Accurate measurement of gas flow in pipelines is important in a variety of situations, such as in the transmission of fuel gases such as natural gas and propane. Fuel gases are typically sold by volumetric measure, so gas flow must be accurately measured and recorded to ensure that customers are charged fully and fairly for the gas delivered to them. Accurate flow measurement is also important for optimum operational control in gas production and processing facilities.
Gas flow measurement and recording are commonly carried out by use of an orifice meter installed in a gas pipeline in conjunction with a circular chart recorder, such as the Models J8, M202, and M208 chart recorders manufactured by Barton Instrument Systems, LLC, of Industry, Calif. An orifice meter works on the venturi principle, in accordance with well-known scientific formulae (specifically, Bernoulli's equation). Its primary feature is an orifice plate, which is a flat plate having a sharp-edged circular or oval orifice that is smaller in diameter than the inner diameter of the pipeline. The orifice plate is installed such that its plane is transverse to the axis of the pipeline, and typically (though not necessarily) with the orifice coaxial with the pipeline. The orifice plate causes a localized constriction of the gas flow, thus causing the gas flow velocity to increase as the gas passes through the orifice, with a resultant drop in pressure on the downstream side of the orifice plate. The gas pressure on each side of the orifice plate is continuously measured by means of upstream and downstream pressure taps closely adjacent to the orifice plate. Because gas temperature is an important factor for accurate calculation of gas flow, the gas temperature is continuously measured upstream of the orifice meter using a temperature sensor such as a resistive temperature device (“RTD”).
The temperature and differential pressure readings are communicated to the chart recorder, which features multiple inkpens that continuously plot the information onto rotating circular paper charts. These charts, which typically record readings over a 7-day period, must be regularly replaced with fresh charts, and the recorded charts must be analyzed and interpreted by skilled technicians to determine the gas flows represented by the information thereon. The calculation of gas flow rates must also take into consideration the particular chemical composition of the gas in question, or, more specifically, the density of the gas.
Natural gas flow calculations are typically required to be made in accordance with analytical methods stipulated by the American Gas Association (“AGA”). Where orifice meters are used, the applicable standards are AGA-3 (for orifice flow calculations), and AGA-8 or NX-19 (to adjust for supercompressibility).
The use of orifice meters and circular chart recorders has a number of practical drawbacks. The accuracy of the gas flow readings is dependent on selection of orifice plates having orifices of appropriate sizes, and this is something that varies with the gas flow rate. Accordingly, it is periodically necessary to change orifice plates to suit variations in gas flow rates. This requirement entails additional labour costs, as does the need for regular gathering and replacement of the circular charts. To these inconveniences must be added the need for periodic adjustment, maintenance, and repair of the inkpens, plus the need to interpret the charts before reliably accurate gas flow measurements can be obtained.
Some of these drawbacks can be overcome by using an electronic flow-measurement device (or “EFM”) in place of a circular chart recorder. Examples of known EFMs include the Daniel® FloBoss™ 103 and FloBoss™ 503 flow computers manufactured by Daniel Measurement and Control Inc., of Houston, Tex. Such EFMs have microprocessors or CPUs (central processing units) that directly calculate gas flows in accordance with AGA-3 and AGA-8 (or NX-19), which are incorporated into the EFM's memory (i.e., as “firmware”). These EFMs provide for digital read-out of instantaneous and historical gas flow rates, and can archive flow calculations covering a period of several weeks, such that this information can be collected at larger and more convenient intervals than would be possible using a chart recorder. Alternatively, and even more advantageously, the flow rate calculations can be transmitted to a remote collection point location, by either hard-wired or wireless data communication links, eliminating or greatly reducing the need for regular visits by field technicians.
It can therefore be seen that EFMs can be used to avoid the drawbacks of circular chart recorders and the interpretation process necessarily associated therewith. However, the disadvantages associated with orifice meters, and in particular the recurring need to replace orifice plates, still remain. These disadvantages may be overcome by using a turbine flow meter instead of an orifice meter.
A turbine meter features a free-wheeling turbine rotor having multiple turbine blades. To measure gas flow, the turbine meter is installed in a gas pipeline with the rotor coaxial with the pipe. The flow of gas in the pipeline causes the turbine rotor to rotate. It is well established that for a given turbine, there is a substantially direct relationship between the number of turbine rotations and the volume of gas flowing past the turbine. It follows that if this relationship has been quantified, the gas flow rate can be easily determined by counting the number of turbine rotations over a selected time interval, and then calculating the flow using fundamental mathematics.
The same result can obviously be achieved by counting partial revolutions corresponding to the angular spacing of the turbine blades, and this is in fact what is almost invariably done. In some common types of turbine meter, the turbine blades are made of a magnetic material (such as mild steel), while the turbine housing is made of a non-magnetic material (such as stainless steel). A sensing element incorporating a permanent magnet is positioned close to but outside the arc of the turbine blades. As each blade passes by the sensor, it interrupts the magnetic field generated by the permanent magnet. The sensor detects these magnetic field interruptions and converts them to electrical pulses, which may be totalized over a selected time interval for purposes of gas flow calculation. In other types of turbine meter, an optical sensor is used to count turbine blade pulses.
The relationship between turbine rotations and gas volume usually varies to some degree with the velocity of the gas (and therefore the flow rate). This phenomenon is taken into account by calibrating each turbine to determine its characteristics over a selected range of pulse frequencies. In accordance with industry standards, this is typically done by passing known volumes of gas through the turbine at various flow rates, to produce a 10-point linearization curve plotting the turbine's “K” factor (the number of pulses per cubic foot of gas) against the pulse frequency (pulses per second). With this information at hand, gas flows can be easily calculated by determining the pulse frequency, determining the “K” factor applicable to that frequency, and then dividing the frequency by the “K” factor, resulting in a value for the gas flow (in cubic feet per second, or other desired units of measurement).
However, accurate gas flow measurement with a turbine meter requires more information than the “K” factor of the turbine; for optimal accuracy, the gas pressure, temperature, and density should also be taken into account. Turbine meters are typically installed in conjunction with EFMs having, in addition to a pulse counter, a pressure transducer, which generates an electronic signal corresponding to the gas pressure upstream of the turbine, and an RTD connection, for reading the gas temperature downstream of the turbine. The gas density is determined by laboratory analysis, and this information is fed into the EFM's data memory. The EFM's CPU can then calculate gas flow rates corrected for these various inputs, in accordance with the appropriate industry standards programmed into the EFM as firmware; i.e., AGA-7 (for turbine meters) and AGA-8 (or NX-19). Examples of known EFM's with these capabilities are the Model BA415R gas computer manufactured by Barton Instrument Systems, and the Daniel® FloBoss™ 504 manufactured by Daniel Measurement and Control Inc.
From the preceding discussion, it can be readily seen that the drawbacks of circular chart recorders can be eliminated by use of EFMs in conjunction with orifice meters, and also that the drawbacks of orifice meters can be eliminated by use of turbine meters in conjunction with suitable EFMs. However, the known EFMs appropriate for use in both of these applications suffer from a significant disadvantage in that they have comparatively large electrical power requirements. The calculations required to be performed in accordance with the various AGA standards are complex, therefore entailing a CPU with substantial computational capacity. As well, the CPU requires very high computing speed in order to produce substantially “real time” flow readings quickly in response to continuous flows of input data from the magnetic pulse sensor, the pressure transducer, and the RTD. The electrical power needed to serve these computational requirements would make battery power impractical, having regard to the current state of battery technology. Therefore, EFMs are typically connected to conventional power sources (e.g., building or plant power), or are installed with dedicated solar panels. Such EFM installations are susceptible to interruption of gas flow data calculation and storage in the event of failure of a conventional power source or physical damage to solar panels due to storms or vandalism.
For the foregoing reasons, there is a need for EFMs that can perform all the functions of known EFMs as described above, in conjunction with either orifice meters or turbine meters, while consuming substantially less electrical power. In particular, there is a need for such EFMs which can operate effectively and efficiently on battery power, and can do so without sacrificing data display and storage capabilities as compared with known EFM that use permanent power sources or dedicated solar panels. The present invention is directed to these needs.